The present invention relates to an array of light-emitting elements, more particularly to an array with a structure that alters the amount of emitted light so that all light-emitting elements to emit the same amount of light, or all emit increased amounts of light.
Arrays of light-emitting elements such as light-emitting diodes (LEDs) are used as light sources in electrophotographic printers. A typical LED array is a semiconductor chip with a row of diffusion regions from which light is emitted when current is supplied. An LED printer may include many of these chips, placed end-to-end to form a single linear array of light-emitting elements.
There is a strong demand for electrophotographic printers with high printing quality, which requires the printing of small, uniform dots with high resolution. For LED printers, this requires a linear array in which the light-emitting elements are closely and uniformly spaced, and have uniform light-emitting characteristics. The uniform spacing requirement includes the spacing between the light-emitting elements at adjacent ends of two different chips.
This last condition forces the light-emitting regions at the two ends of each chip to be very close to the edges of the chip. A consequent problem is that some of the light generated in these light-emitting elements is reflected or scattered by the edges of the chip and fails to be emitted in a useful direction. This problem becomes evident in very high-resolution printers, leading to faint vertical lines of under-sized dots.
Other problems encountered in the LED arrays used in printers include high cost and high power consumption. The present inventors (and others) have shown that costs can be reduced by dividing the array into blocks and employing a multi-layer wiring scheme that lessens the necessary number of wire-bonding pads. Power consumption in this wiring scheme can be reduced by placing common electrodes, which are located on the surface of the array, close to the light-emitting regions, so that the applied voltage can be reduced. A solution to the problem of reduced light emission at the ends of the array that is also applicable to this multi-layer wiring scheme is highly desirable.
Needless to say, it is also desirable to increase the amount of light emitted by the light-emitting elements, so that printing speed can be increased.
An object of the present invention is to compensate for reduced light emission at the two ends of an array of light-emitting elements, so that all light-emitting elements in the array emit the same amount of light.
Another object of the invention is to increase the amount of light emitted by the light-emitting elements in the array.
The invented array of light-emitting elements includes a semiconductor layer of a first conductive type, in which a plurality of light-emitting regions of a second conductive type are formed. The array also has at least one emission-altering element, separated from all of the light-emitting regions, altering an amount of light emitted from an adjacent one of the light-emitting regions.
The emission-altering element may be a region with a trench disposed between two mutually adjacent light-emitting regions, extending at least partway into the semiconductor layer between two adjacent light-emitting regions, or an opaque member disposed between two mutually adjacent light-emitting regions. Alternatively, the emission-altering element may be a non-emitting region of the second conductive type, disposed either between two mutually adjacent light-emitting regions or at an edge of the array.
A trench or opaque member decreases the amount of light emitted by the adjacent light-emitting regions. Emission-altering elements of these types, placed between every second pair of mutually adjacent light-emitting regions, or between every pair, can make the light-emitting regions in the interior of the array emit the same amount of light as the light-emitting regions at the ends of the array.
A non-emitting region of the second conductive type increases the amount of light emitted by the adjacent light-emitting regions. Placed at an edge of the array, the non-emitting region can make the light-emitting region nearest the edge emit the same amount of light as light-emitting regions in the interior of the array. Placed between adjacent light-emitting regions, the non-emitting region can increase the optical output of the array.
If the array has multi-layer wiring and the semiconductor layer is divided into blocks, some of the emission-altering elements can also provide electrical isolation between the blocks.